1. Field of the Invention
This invention relates to a conductive elastic member for electrophotography, and more particularly to a roller-shaped conductive elastic member, and also to a process for manufacturing thereof, a mass production process therefor, a process cartridge, and an electrophotographic apparatus.
2. Related Art
In the following, the present invention is described in detail taking the case of the charging member, in particular, a roller-shaped charging member, such as charging roller, as one example, but the electrophotographic conductive or semiconductive elastic member of the present invention may be any member as long as it is a member used in electrophotography and required to have conductivity or semiconductivity, and elasticity. It may include, besides the charging member, e.g., a developing member, a transfer member, a charge elimination member, and a transport member such as a paper feed roller.
In electrophotographic apparatus, a contact charging system is employed as one of ways for charging the surfaces of electrophotographic photosensitive members electrostatically. The contact charging system is a system where a charging member to which a voltage is kept applied is brought into proximity to or contact with the surface of an electrophotographic photosensitive member to charge the surface of the electrophotographic photosensitive member electrostatically. As a charging member therefor, commonly used is a roller-shaped charging member (charging roller) constituted of a mandrel made of a metal and a conductive (semiconductive) elastic layer covering around the mandrel.
The elastic layer of the charging roller is required to have an appropriate conductivity in order to prevent a leak from being caused by pinholes, scratches or the like of the electrophotographic photosensitive member surface. In order for the electrophotographic photosensitive member to be uniformly charged, it is also important for the elastic layer to be conductive (semiconductive) to have an electrical resistance value of from 1×103 to 1×1010 Ω·cm as volume resistivity.
In order for the elastic layer to hold an appropriate conductivity, conductive particles are dispersed in a rubber which is a material constituting the elastic layer, or a conductive rubber is used in which the rubber itself has conductivity.
Now, the elastic layer of the charging roller is produced using a rubber composition containing such a rubber with conductive particles dispersed therein, or the conductive rubber, and a vulcanizing agent and optionally containing a vulcanization accelerator and so forth, and by extruding the rubber composition into a roller, e.g., in such a way that it covers the peripheral surface of a mandrel, and thereafter vulcanizing the rubber. However, air bubbles (hereinafter “voids”) may be produced in the elastic layer as a result of the evaporation of water incorporated in the rubber or rubber composition, caused by the heat at the time of vulcanization. Then, when the surface of the elastic layer of this charging roller is polished, such voids may come exposed to the elastic layer surface to make the elastic layer have concavities on its surface.
Where such a charging roller is set in an electrophotographic apparatus and images are reproduced to make evaluation, the part having such defects may cause faulty charging to cause faulty images. Such faulty images caused by voids may occur Without regard to whether or not the elastic layer surface has been subjected to modification treatment such as irradiation with ultraviolet light, irradiation with electron rays or impregnation with a surface treatment solution. Also, especially where a surface coat layer is formed on the elastic layer by coating, even if the voids standing exposed to the elastic layer surface are microscopic, their presence on the elastic layer surface may cause large crater-like defects on the surface coat layer which have grown from the voids as nuclei. Then, such voids may remarkably occur especially when using an ion-conductive rubber having a polar group in the molecule. This is considered due to the fact that the polar group tends to adsorb water molecules.
To solve such problems, a method is proposed in which a rubber is compounded with a dehydrating agent which removes water content of unvulcanized rubbers, as exemplified by calcium oxide (see, e.g., Japanese Patent Application Laid-Open No. H9-297454). However, the dehydrating agent has a poor dispersibility in rubbers, and also the compounding of rubber with the dehydrating agent may make the elastic layer have a high hardness.
Accordingly, the present inventor has devised that, when a rubber composition having not been vulcanized is extruded by means of a vented extruder, the extrusion temperature is set high so as to remove water content and other volatile components in the unvulcanized rubber. In such a case, however, depending on the type of a vulcanization accelerator in the rubber composition, vulcanization may unintentionally begin to take place at the time of extrusion to make the unvulcanized rubber begin to cure (hereinafter “scorch” or “scorching”), making it difficult to carry out extrusion.
Meanwhile, as long as the elastic layer extruded into a roller has sufficiently small surface roughness, the elastic layer need not to be polished, and because of that, there is less possibility that concavities are formed on the elastic layer because of the voids even if any voids are present in the elastic layer. However, in order to form such an elastic layer having small surface roughness, it is necessary, e.g., to extrude an unvulcanized rubber composition at a high temperature, where it has been unavoidable to cause scorching at the time of extrusion and various difficulties coming incidentally thereto.
Taking account of productivity of charging members, it is indispensable to use a vulcanization accelerator in order to shorten the time necessary for vulcanization. The present inventor has perceived that, in order to efficiently manufacture high-grade conductive elastic members used for electrophotography, it is necessary to find out a vulcanization accelerator that can satisfy at a high level the incompatible properties that it can not easily cause the scorching at the time of extrusion and has high vulcanization rate.
As vulcanization accelerators used to vulcanize conductive rubber materials used in electrophotographic apparatus, various examples are enumerated in, e.g., Japanese Patent Application Laid-Open No. 2000-265008. According to studies made by the present inventor, however, it has been unable to find out vulcanization accelerators that can not easily cause any scorching at the time of high-temperature extrusion and have relatively high vulcanization rate, and also can provide vulcanized rubbers with superior quality. It has also been found that a sulfenamide type vulcanization accelerator, which is commercially available as a vulcanization accelerator having relatively high vulcanization rate, makes rubbers vulcanized therewith worse in its permanent set and found difficult to obtain high-grade elastic layers.